Chemical process

ABSTRACT

Processes for preparing compounds of Formula I and IV 
     
       
         
         
             
             
         
       
     
     are described.

This application is a Continuation of U.S. application Ser. No. 10/598,116 (filed Aug. 17, 2006), which is a US National Stage under 35 U.S.C §371 of International Application No. PCT/GB2005/000567 (filed Feb. 17, 2005),which claims priority under 35 U.S.C. §119(a)-(d) to Application No. GB 0403744.6 filed on Feb. 20, 2004.

The present invention relates to an improved chemical process for preparing intermediates. Certain of these intermediates are useful in the manufacture of compounds which are useful in the treatment of, for example, cancer, pain and cardiovascular diseases in a warm-blooded animal such as man, particularly compounds which possess endothelin receptor antagonist activity.

In particular, the present invention relates to a chemical process for preparing [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid which is used in the manufacture of N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide which compound is disclosed as Example 36 of International Patent Application WO96/40681. This compound possesses endothelin receptor antagonist activity, and accordingly is useful whenever such antagonist activity is desired, such as for research tools within pharmacological, diagnostic, and related studies or in the treatment of diseases and medical conditions including, but not limited to hypertension, pulmonary hypertension, cardiac or cerebral circulatory disease and renal disease. In addition this compound is also useful in the treatment of cancer and pain, in a warm-blooded animal such as man.

A route for preparing N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide is disclosed in International Patent Applications WO 96/40681 and WO 98/40332. The route involves the use of the compound N-(isobutoxycarbonyl)-2-(4-methoxycarbonylphenyl)-N-(3-methoxy-5-methylpyrazin-2-yl)pyridine-3-sulphonamide as an intermediate with the formation of the 1,3,4-oxadiazole in the 4-position of the phenyl group occurring at the end of the synthesis. This existing route is satisfactory for the synthesis of relatively small amounts of N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide but is a linear rather than convergent synthesis, involving the isolation of a substantial number of intermediates. As such, the overall yield of this synthesis is not high.

Furthermore, as the heteroaryl moiety at the 4-position of the phenyl group is formed as the last step, it is necessary to undergo a linear synthesis approach with the rest of the molecule made first. This is clearly undesirable when substituents in distinct parts of the molecule need to be varied in order to investigate structure-activity relationships. It would be highly desirable if a convergent approach to the synthesis of this type of compound could be devised. This would also be of significant benefit in the efficiency of manufacturing large scale amounts of N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide.

We have now devised a much improved process for the manufacture of heteroaryl-phenyl boronic acids, in particular, [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid. The process allows exploitation of a more convergent route to N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide than the previously described route and allows a reduction in the number of intermediates that must be isolated. This provides significant advantages of time and cost of manufacture.

In a further aspect of the present invention one of the heteroaryl-phenyl boronic acids, [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid, produced according to the present invention, is used to prepare N-protected N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamides, in particular N-(isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide. These intermediates may then be deprotected to form N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide.

The process for the manufacture of heteroaryl-phenyl boronic acids of the present invention utilises the increased acidity of the heteroaryl ring proton, and involves the sequential use of two bases. Initial attempts at adding one equivalent of a base to a heteroaryl-phenyl bromo compound in order to induce halogen-metal exchange led to competing deprotonation of the heteroaryl ring. On quenching with a borate ester, a negligible yield of the desired product was achieved, together with starting material and by-products. The present inventors found, surprisingly, that the sequential use of two bases leads to good yields of the desired heteroaryl-phenyl boronic acids. In the process of the present invention the heteroaryl ring is initially deprotonated with a (typically) “weaker” base, before inducing halogen-metal exchange with a (typically) “stronger” base.

According to a first aspect of the present invention, there is provided a process for the preparation of a compound of the Formula I

wherein,

-   X₁ is selected from O, NR₁ or S; and -   X₂ is selected from CH or N; -   wherein R₁ is a nitrogen-protecting group, -   which comprises:— -   the sequential reaction of a compound of the Formula II

with,

(i) methyl- or an optionally substituted aryl-lithium; and then

(ii) n-butyl-, s-butyl-, t-butyl- or n-hexyl-lithium; and then

(iii) a borate ester.

For process steps (i), (ii) and (iii), the reactions may conveniently be carried out in an inert solvent or diluent or an ethereal solvent such as diethyl ether, tetrahydrofuran, diethoxymethane, 1,2-dimethoxyethane or 1,4-dioxan. Thus, for example, the reaction may be carried out by sequentially treating 2-(4-bromophenyl)-1,3,4-oxadiazole with 4-methylphenyllithium, followed by n-hexyllithium, and finally triisopropylborate in a suitable solvent or diluent, for example, an ethereal solvent such as tetrahydrofuran, at a temperature in the range, for example, −90 to −50° C., more particularly −70 to −55° C., conveniently at or near −70° C.

Optionally the heteroaryl-phenyl bromo compound of Formula II can be charged to a solution of the first base to enable deprotonation, followed by the addition of the second base to induce transmetallation. This method although slightly less efficient in yield and quality does have advantages in cases where the first base must be generated in situ due to lack of stability at ambient temperatures. In this case only one cryogenic vessel is required to complete the processing.

The molar ratios of the reagents used in process steps (i), (ii), and (iii), are preferably in the range from 1.0-1.5: 1.0-1.5: 2.1-3 respectively, but more preferably in the range 1.06-1.3: 1.07-1.1: 2.2-2.3 respectively. Conveniently, the lithiated intermediates formed during the conversion of compounds of the Formula II to compounds of Formula I are not isolated as such but are each prepared and used as a solution in an organic solvent. Thereby, compounds of Formula I may be manufactured from compounds of Formula II in a one-pot procedure.

An aryl lithium is, for example, phenyl or naphthyl-lithium.

An optional substituent for an aryl lithium is, for example, methyl.

Particularly preferred optionally substituted aryl lithiums are, for example, phenyl-, 2-methylphenyl-, 4-methylphenyl-, mesityl- or naphthyl-lithium.

A borate ester is an alkyl, alkenyl or aryl boronic ester, for example, trimethyl-, triethyl- or triisopropyl-borate.

When R₁ is a nitrogen-protecting group, then, for example, suitable methods for protection are those known to those skilled in the art. Conventional protecting groups may be used in accordance with standard practice (for illustration see T. W. Green, Protective Groups in Organic Synthesis, John Wiley and Sons, 1991).

A suitable nitrogen- protecting group, R₁, is, for example, an (1-6C)alkyl, phenyl, allyl, methoxymethyl, benzyl, triphenylmethyl or diphenylphosphinyl protecting group.

This first aspect of the present invention provides compounds of Formula I in commercially acceptable yields and of high quality.

Further values of X₁ and X₂ are as follows. Such values may be used where appropriate with any definitions, claims or embodiments defined hereinbefore or hereinafter.

X₁ is O.

X₁ is NR₁

X₁ is S.

X₂ is CH.

X₂ is N.

X₁ is O, and X₂ is CH.

X₁ is O, and X₂ is N.

X₁ and X₂ are N.

X_(i) is NR₁, and X₂ is CH.

X₁ is NR₁, and X₂ is N.

X₁ is S and X₂ is CH.

X₁ is S and X₂ is N.

R₁ is allyl or benzyl.

R₁ is benzyl.

Therefore in an additional aspect of the invention there is provided a process for the preparation of compounds of the Formula I

wherein,

-   X₁ is selected from O, NR₁ or S; and -   X₂ is selected from CH or N; -   wherein R₁ is a nitrogen-protecting group; -   which comprises:— -   the sequential reaction of compounds of the Formula II

with,

(i) 4-methylphenyllithium; and then

(ii) n-hexyllithium; and then

(iii) triisopropylborate.

In a further aspect of the invention there is provided a process for the preparation of compounds of the Formula I

wherein,

-   X₁ is selected from O, NR₁ or S; and -   X₂ is selected from CH or N; -   wherein R₁ is a nitrogen-protecting group; which comprises:— -   the sequential reaction of compounds of the Formula II

with,

(i) methyllithium; and then

(ii) n-hexyllithium; and then

(iii) triisopropylborate.

In a further aspect of the invention there is provided a process for the preparation of compounds of the Formula I

wherein,

-   X₁ is O; and -   X₂ is N; -   which comprises:— -   the sequential reaction of compounds of the Formula II

with,

(i) methyllithium; and then

(ii) n-butyllithium; and then

(iii) triisopropylborate.

In a further aspect of the invention there is provided a process for the preparation of compounds of the Formula I,

wherein,

-   X₁ is O; and -   X₂ is N; -   which comprises:— -   the sequential reaction of compounds of the Formula II

with,

(i) 4-methyphenyllithium; and then

(ii) n-butyllithium; and then

(iii) triisopropylborate.

Compounds of the formula (II) may be prepared according to the experimental methods and procedures disclosed in Bioorganic & Medicinal Chemistry Letters, 2002, 12(20), 2879-2882; Eur. J. Med. Chem., 2000, 35, 157-162; Helvetica Chimica Acta, 1950, 33, 1271-1276; Eur. J. Med. Chem., 1985, 20(3), 257-66 and J. Het. Chem., 1989, 26, 1341.

A further aspect of the present invention provides the use of [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid, prepared according to the present invention, in the preparation of compounds of Formula IV which are intermediates useful in the preparation of N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide.

N-(3-Methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide is prepared by deprotecting compounds of Formula IV.

In this aspect of the invention [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid is coupled with compounds of Formula III to form compounds of Formula IV.

Particularly this reaction takes place in an aqueous solvent, for example methanol, ethanol, isopropanol, industrial methylated spirit (IMS), isobutanol, NMP (N-methylpyrrolidinone), DMF; with or without an organic phase, for example toluene or xylenes at a temperature in the range, for example 60 to 100° C. more particularly 75 to 85° C., in the presence of:

-   -   (i) the boronic acid     -   (ii) a suitable source of palladium (0), for example PdCl₂,         Pd(Ph₃P)₄ or Pd(OAc)₂;     -   (iii) a suitable ligand, for example triphenylphosphine or         3,3′,3″-phosphinidyne tris(benzenesulphonic acid) trisodium         salt;     -   (iv) a base, for example triethylamine, benzyldimethylamine,         N-methylmorpholine, N-methylpiperidine, triethanolamine,         ethyldiethanolamine, diisopropylethylamine, potassium acetate,         cesium fluoride or potassium fluoride.

Particularly the source of palladium is palladium acetate.

Particularly the base is N-methylmorpholine. In another aspect, particularly the base is triethylamine.

Particularly this reaction takes place in an aqueous solvent without an organic phase. In another aspect, particularly this reaction takes place in an aqueous solvent with an organic phase. Where this reaction takes place in an aqueous solvent with an organic phase, particularly the organic phase comprises toluene. In another aspect of the present invention, where this reaction takes place in an aqueous solvent with an organic phase, particularly the organic phase comprises xylene.

In another aspect, this reaction more specifically takes place in the presence of palladium acetate, 3,3′,3″-phosphinidyne tris(benzenesulphonic acid) trisodium salt, N-methylmorpholine in water and isopropanol.

In another aspect, this reaction more specifically takes place in the presence of palladium acetate, 3,3′,3″-phosphinidyne tris(benzenesulphonic acid) trisodium salt, triethylamine, xylene, water and IMS.

The molar ratios of the reagents used in process steps (i), (ii), (iii) and (iv), are preferably in the range from 1.0-2.0: 0.02-0.3: 0.06-0.9: 1.5-5.0 respectively, but more preferably in the range 1.4-1.6: 0.03-0.1: 0.09-0.3: 2.0-3.0 respectively.

In compounds of Formula III or Formula IV, P is a nitrogen-protecting group. Suitable methods for protection are those known to those skilled in the art. Conventional protecting groups may be used in accordance with standard practice (for illustration see T. W. Green, Protective Groups in Organic Synthesis, John Wiley and Sons, 1991).

A suitable value for P is, for example, an acyl group, for example a C₁₋₆alkanoyl group such as acetyl; an aroyl group, for example benzoyl; a C₁₋₆alkoxycarbonyl group, for example a methoxycarbonyl, ethoxycarbonyl, isobutoxycarbonyl or tert-butoxycarbonyl group; an arylmethoxycarbonyl group, for example benzyloxycarbonyl; a phosphinyl group, for example diphenylphosphinyl; a benzyl group or a C₂₋₆alkenyl group such as allyl.

A suitable value for P is a C₁₋₆alkoxycarbonyl group. More suitable values for P are a methoxycarbonyl, ethoxycarbonyl or isobutoxycarbonyl group. More specifically a value for P is isobutoxycarbonyl.

The deprotection conditions for the nitrogen protecting groups described herein necessarily vary with the choice of protecting group. Thus, for example, an acyl group such as a C₁₋₆alkanoyl or a C₁₋₆alkoxycarbonyl group or an aroyl group may be removed for example, by hydrolysis with a suitable base such as an alkali metal hydroxide, for example lithium or sodium hydroxide or an amine, for example ammonia. Alternatively an alkoxycarbonyl group such as a t-butoxycarbonyl group may be removed, for example, by treatment with a suitable acid as hydrochloric, sulphuric or phosphoric acid or trifluoroacetic acid and an arylmethoxycarbonyl group such as a benzyloxycarbonyl group may be removed, for example, by hydrogenation over a catalyst such as palladium-on-carbon, or by treatment with a Lewis acid for example boron tris(trifluoroacetate). A phosphinyl group may be removed by base hydrolysis such as an alkali metal hydroxide, for example lithium or sodium hydroxide or an amine, for example ammonia. A benzyl group may be removed by hydrogenation over a catalyst such as palladium-on-carbon. A C₂₋₆alkenyl group such as allyl may be removed palladium assisted hydrolysis.

In a further aspect of the invention there is provided a process for preparing a compound of Formula IV which comprises reacting [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid with a compound of Formula III.

In a further aspect of the invention there is provided a process for preparing a compound of Formula IV which comprises reacting [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid, prepared according to the present invention, with a compound of Formula III.

In this aspect of the invention, more specifically the invention provides the use of [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid, prepared according to the present invention, in the preparation of N-(isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide a compound of Formula IV and an intermediate useful in the preparation of N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide.

In this aspect of the invention [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid is coupled with N-(isobutoxycarbonyl)-2-chloro-N-(3-methoxy-5-methylpyrazin-2-yl)pyridine-3-sulphonamide to form N-(isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide.

The preparation of N-(isobutoxycarbonyl)-2-chloro-N-(3-methoxy-5-methylpyrazin-2-yl)pyridine-3-sulphonamide is described in Example 1 of WO96/40681.

Thus according to this aspect of the invention there is provided a process for preparing N-(isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide which comprises coupling [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid with N-(isobutoxycarbonyl)-2-chloro-N-(3-methoxy-5-methylpyrazin-2-yl)pyridine-3-sulphonamide.

Therefore in a further aspect of the invention there is provided the use of [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid in the preparation of N-(isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide.

In a further aspect of the invention there is provided the use of [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid, prepared according to the process of the present invention, in the preparation of N-(isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide.

In a further aspect of the invention there is provided a compound of Formula IV.

In a further aspect of the invention there is provided N-(isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide.

In a further aspect of the invention there is provided the use of N-(isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide in the preparation of N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide.

The invention will now be illustrated by the following non-limiting Examples in which, unless otherwise stated:—

-   (i) yields are intended for the assistance of the reader only and     are not necessarily the maximum attainable by diligent process     development; -   (ii) ¹H NMR spectra were determined at either 270 MHz or 400 MHz in     DMSOd₆ using tetramethylsilane (TMS) as an internal standard, and     are expressed as chemical shifts (delta values) in parts per million     relative to TMS using conventional abbreviations for designation of     major peaks: s, singlet; m, multiplet; t, triplet; br, broad; d,     doublet.

EXAMPLE 1 [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid

A solution of methyllithium (8% w/w in diethoxymethane) (65 ml) was added to a suspension of 2-(4-bromophenyl)-1,3,4-oxadiazole (40 g) in tetrahydrofuran (THF) (415 ml) at −65° C. After an hour a solution of n-butyllithium (2.5M in hexanes) (78 ml) was then added at −65° C. After an hour, triisopropylborate (90 ml)) was then added maintaining the reaction mixture at −65° C. The reaction mixture was held at −65° C. for an hour and then warmed to −20° C. and drowned out into a mixture of acetic acid (28 ml) in water (222 ml). The resultant solid was isolated, washed with THF and water, and dried to yield the title compound (28.96 g@95.1% w/w, 82%); 400 MHz NMR Spectrum: (DMSOd₆) 8.00 (s, 4H), 8.31 (s, 2H), 9.35 (s, 1H); Mass Spectrum MH⁺ 191.0628 (calc. using 11-B) Found 191.0633.

The 2-(4-bromophenyl)-1,3,4-oxadiazole used as a starting material was prepared as follows:

To a suspension of 4-bromobenzoic hydrazide (200 g) in industrial methylated spirit (700 ml) was added triethylorthoformate (309 ml), industrial methylated spirit (100 ml) and sulphuric acid (0.8 m1). The reaction mixture was heated to reflux for 1 hour. The reaction mixture was cooled to 0-5° C. and product crystallised. Product was isolated, washed and dried to yield 2-(4-bromophenyl)-1,3,4-oxadiazole (186.1 g, 89.9%). 400 MHz NMR Spectrum: (DMSOd₆) 9.35 (s, 1H), 7.98 (d, 1H), 7.95 (d, 1H), 7.84 (d, 1H), 7.81 (d, 1H); Mass Spectrum MH⁺ 224.9663 (calc. using 79-Br) Found 224.9701.

EXAMPLE 2 [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid

Lithium granules (8.2 g) and tetrahydrofuran (670 g) were charged to a reactor under an argon atmosphere and the mixture cooled to −35° C. 4-Chlorotoluene (74.3 g) was added at −35° C. 30 and the mixture was held at this temperature for 6 hours. The resultant solution was added to a suspension of 2-(4-bromophenyl)-1,3,4-oxadiazole (124.4 g) in tetrahydrofuran (800 g) at −65° C. After 30 mins a solution of n-hexyllithium (33% w/w in hexanes) (240 ml) was then added at −65° C. After a further 30 min triisopropylborate (230.8 g) was then added maintaining the reaction mixture at −65° C. The reaction mixture was allowed to warm to −35° C. and drowned out into a solution of acetic acid (91.5 g) in water (688 g). The resultant solid was isolated, washed with THF and water, and dried to yield the title compound (92.2 g, 88%).

EXAMPLE 3 [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid

Example 2 was repeated but the charge of 4-chlorotoluene increased from 1.06 moles to 1.30 moles. The yield of the title compound increased to 89.3%.

EXAMPLE 4 [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid

Tetrahydrofuran (250 g) was charged to a mixture of lithium granules (3.02 g) and biphenyl (0.01 g) under an argon atmosphere and the mixture cooled to −30° C. 2-Chlorotoluene (27.55 g) was slowly added at −30° C. The reaction was held at −30° C. for 6 hours and then cooled to −65° C. A mixture of 2-(4-bromophenyl)-1,3,4-oxadiazole (50.0 g) in THF (300 g) was slowly added at −65° C. The reaction was held at −65° C. for 30 minutes then a solution of n-hexyllithium (33% w/w in hexanes, 86 ml) was added at −65° C. The reaction was held at −65° C. for 30 minutes and then trimethylborate (48.7 g) was added at −65° C. The reaction was held at −65° C. for 10 minutes then methanol (55.3 g) was added followed by 4-methyl-2-pentanone (240 g). The reaction mixture was warmed and the low boiling solvents distilled off under vacuum to a maximum temperature of 55° C. The residual mixture was cooled to 0° C. and 10% w/w sulphuric acid (92 g) was added followed by water (92 g) whilst maintaining the temperature below 7° C. Product precipitated. The pH was adjusted to 6.5 by the addition of more 10%w/w sulphuric acid (85.3 g). The mixture was heated to 40° C. then cooled back to 5-10° C. Product was isolated and washed with THF (56 g) and water (60 g), yielding wet title compound (25.2 g, 60%).

EXAMPLE 5 [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid

Tetrahydrofuran was charged to lithium granules (7.6 g) under an argon atmosphere and the mixture cooled to −30° C. 2-Chlorotoluene (69.4 g) was slowly added at −30° C. The reaction was held at −30° C. for 6 hours then added to a suspension of 2-(4-bromophenyl)-1,3,4-oxadiazole (124.4 g) in tetrahydrofuran (800 g) at −65° C. The reaction was held at −65° C. for 30 minutes then a solution of n-hexyllithium (33% w/w in hexanes, 245 ml) was added at −65° C. The reaction was held at −65° C. for 30 minutes and then trimethylborate (230.8 g) was added at −65° C. The reaction was held at −65° C. for 30 minutes then methanol (175 ml) was added followed by 4-methyl-2-pentanone (600 g). The reaction mixture was warmed and the low boiling solvents distilled off under vacuum to a maximum temperature of 50° C. The reaction mixture was cooled to 5-10° C. and the pH adjusted to 6.5 by the addition of 5% w/w sulphuric acid (990.5 g). Product precipitated. The mixture was heated to 40° C. then cooled back to 10° C. Product was isolated, washed with THF and water, and dried yielding the title compound (79.3 g, 75.5%).

EXAMPLE 6 [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid

Example 4 was repeated but chlorobenzene (61.6 g) was used instead of 2-chlorotoluene. The isolated yield of the title compound was 87.8 g, (83.8%).

EXAMPLE 7 N-(Isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide

Palladium acetate (0.4144 g) and 3,3′,3″-phosphinidyne tris(benzenesulphonic acid) trisodium salt 30% w/w aq sol (3.26 g) were dissolved in water (35 ml) over 6 minutes in an ultrasonic bath. The yellow solution was added to a stirred slurry of [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid (10 g) and isobutyl [(2-chloropyridin-3-yl)sulfonyl](3-methoxy-5-methylpyrazin-2-yl)carbamate (16.86 g) in xylene (100 ml), industrial methylated spirit (50 ml) and triethylamine (17 ml). The catalyst dissolution flask was then washed in with water (5 ml) and the reaction mixture heated to reflux (80° C.) on an oil bath (105° C.) and stirred at reflux for 24.5 hours. The reaction mixture was cooled to 30° C. and filtered through a Whatman GF/B glass filter paper and the lower aqueous phase separated off. The reaction flask and filter cake was washed with xylene (20 ml). The xylene wash was used to re-extract the aqueous phase. The combined organic phases were stirred and heated to reflux (85° C.) in a clean 500 ml 4-necked flask equipped with overhead stirrer, water condenser, and nitrogen atmosphere. Essochem solvent 30 (hydrocarbons Bp 100-130° C.) (100 ml) was added dropwise over 6 min and the mixture was allowed to self cool to ambient temperature and then further cooled to −5° C. and held for 1 hour. The product was filtered off and washed with Essochem solvent 30 (50 ml). The cake was dried on the filter for 3 hours to give 15.20 g@100% strength, yield 76.8%. 270 MHz ¹H-NMR Spectrum: 0.70 (d, 6H), 1.72 (m, 1H), 2.51 (s, 3H), 3.84 (d, 2H), 4.00 (s, 3H), 7.59 (m, 1H); 7.80 (d, 2H), 7.90 (s, 1H), 8.17 (d, 2H), 8.50 (s, 1H), 8.90 (m, 1H) and 9.00 (d, 1H). Mass Spectrum MH⁺=525.2 (C₂₄H₂₅N₆O₆S=525.16).

EXAMPLE 8 N-(Isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide

To a nitrogen purged 500 mL multi necked flask equipped with an overhead stirrer was charged isobutyl [(2-chloropyridin-3-yl)sulfonyl](3-methoxy-5-methylpyrazin-2-yl)carbamate (22.15 g), [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid (12.26 g), isopropanol (60 ml), water (140 ml) and 3,3′,3″-phosphinidyne tris(benzenesulphonic acid) trisodium salt 30% w/w aq sol (13.7 g). Agitation was started and palladium acetate (0.541 g) was added after 10 minutes. N-Methylmorpholine (13.25 ml) was added and the temperature was adjusted to 80° C. After 4 h 20 min toluene (140 ml) was added and the temperature adjusted to 60° C. After a further 45 min the mixture was filtered through a 1 μm glass fibre filter paper and the aqueous phase separated off. The reaction flask and filter cake was washed with toluene (22 ml). The toluene wash was used to re-extract the aqueous phase and the organic layers were combined. These contained the title compound (22.8 g, 90%) which was not isolated.

EXAMPLE 9 N-(Isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide

To a nitrogen purged 150 ml multi necked flask equipped with an overhead stirrer was charged isobutyl [(2-chloropyridin-3-yl)sulfonyl](3-methoxy-5-methylpyrazin-2-yl)carbamate (7.75 g), [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid (4.29g), isopropanol (21 ml), water (49 ml) and 3,3′,3″-phosphinidyne tris(benzenesulphonic acid) trisodium salt 30% w/w aq sol (2.88 g). Agitation was started and palladium acetate (0.114 g) was added after 10 minutes. Potassium fluoride (2.48 g) was added and the temperature was adjusted to 80° C. After 5 h toluene (49 ml) was added and the temperature adjusted to 60° C. After a further 10 min the mixture was filtered through a 1 μm glass fibre filter paper and the aqueous phase separated off. The organic phase contained the title compound (7.36 g, 83%) which was not isolated. 

1. A process for the preparation of a compound of the Formula I

wherein, X₁ is selected from O, NR₁ or S; and X₂ is selected from CH or N; wherein R₁ is a nitrogen-protecting group, which comprises:— the sequential reaction of a compound of the Formula II

with, (i) methyl- or an optionally substituted aryl-lithium; and then (ii) n-butyl-, s-butyl-, t-butyl- or n-hexyl-lithium; and then (iii) a borate ester.
 2. The process according to claim 1 wherein X₁ is O.
 3. The process according to claim 1 wherein X₂ is N.
 4. The process according to claim 1 wherein said methyl- or an optionally substituted aryl-lithium is 4-methylphenyllithium or methyllithium.
 5. The process according to of claim 1 wherein said n-butyl-, s-butyl-, t-butyl- or n-hexyl-lithium is n-hexyllithium or n-butyllithium.
 6. The process according to claim 1 wherein said borate ester is triisopropylborate.
 7. [4-(1,3,4-Oxadiazol-2-yl)phenyl]boronic acid prepared by the process as claimed in claim
 1. 8. A process for preparing compounds of Formula IV:

which comprises coupling [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid with a compound of Formula III:

wherein P is a nitrogen-protecting group.
 9. The process according to claim 8 which takes place in the presence of (i) a source of palladium (0) selected from PdCl₂, Pd(Ph₃P)₄ or Pd(OAc)₂; (ii) a suitable ligand selected from triphenylphosphine or 3,3′,3″-phosphinidyne tris(benzenesulphonic acid) trisodium salt; (iii) a base selected from triethylamine, benzyldimethylamine, N-methylmorpholine, N-methylpiperidine, triethanolamine, ethyldiethanolamine, diisopropylethylamine, potassium acetate, cesium fluoride or potassium fluoride.
 10. The process according to claim 8 wherein said [4-(1,3,4-oxadiazol-2-yl)phenyl]boronic acid is prepared according to the process as claimed in of claim
 1. 11. The process according to claim 8 wherein P is isobutoxycarbonyl.
 12. A compound of Formula IV:

wherein P is a nitrogen-protecting group.
 13. A compound of Formula IV as claimed in claim 11 which is N-(isobutoxycarbonyl) N-(3-methoxy-5-methylpyrazin-2-yl)-2-(4-[1,3,4-oxadiazol-2-yl]phenyl)pyridine-3-sulphonamide. 